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RIP Stability Features - Hop Count Limits, Hold-Downs, Split Horizons

This issue and the routing loop will continue indefintely until some external boundary condition is reached. That condition is RIP's maximum hop-count. When hop count exceeds 15, the route is marked unreachable and is removed over a period of time.


Hold-Downs

Hold-downs prevent inappropriately reinstating a route that has gone bad when routers broadcast their regular update messages.

When a route is down, neighbour routers will detect it and attempt to broadcast route changes after they have calculated the new routes. This triggered route updates may not arrive at certain network devices and those devices may broadcast a regular update message stating that the route that has gone down is still good to devices that has just been notified of the network failure.

As such, the latter devices contains incorrect routing information which they may potentially further advertise.

Hold downs tell routers to hold on to any changes that might affect recently removed routes for a certain period of time, usually calculated just to be greater than the period of time necessary to update the entire network with a route change. This prevents count-to-infinity problem.


Split Horizons

It is never useful to send information about a route back in the direction from which it came and thus split horizons is used to prevent updates that are redundant to the network.

An example would be Router 1 which has a route to Network A advertises it. If Router 2 is sending traffic to Network A via Router 1, there is no reason for Router 2 to include the route info in its update back to Router 1 because Router 1 is closer to Network A.

Without split horizon rule in place, Router 2 would continue to inform Router 1 that it can actually get to Network A through 2 hops which is via Router 1. If there is a failed direct connection to Network A, Router 1 may direct traffic to Router 2 thinking its an alternative route to Network A and thus causing a routing loop.

Split horizon in this instance serve as an extra algorithm stability.


Routing Information Protocol, Routing Table Format
Routing Table Format, Stability Features - Hop-Count Limit
Stability Features - Hop Count Limits, Hold-Downs, Split Horizons
Stability Features - Poison Reverse Updates, RIP Version 2 (RIPv2)